1. Field of the Invention
This invention generally relates to optical multimeters and more particularly to signal generating portions thereof.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DSO and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. The data structure of a basic SONET signal at a typical data rate of 51.84 Mbps, a.k.a. an STS-1 signal, has 9 rows of 90 columns of 8 bit bytes at 125 xcexcs frame period. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes that are used for various control purposes. The remaining 87 columns of bytes constitute the STS-1 synchronous payload envelope (SPE). As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and coherency correction by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are detected by photodetectors separated into channels, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for optical multimeters, e.g. signal generators and detectors, which can be used to test the various components of an optical, e.g. SONET, network. What is needed is a tunable optical signal generator that does not require the complex control systems relied on by prior art devices. Those control systems utilize closed loop feedback of wavelength or position to select the output wavelength of the optical signal generator. As a result they are expensive and exhibit a large form factor.
The present invention provides an apparatus for passively stabilizing the optical pathlength in tunable lasers. Lasers stabilized using the passive stabilization apparatus exhibit reduced mode hop and increased wavelength stability during temperature variations of the laser or surrounding environment. The stabilization makes the laser suitable for a broad range of applications including optical signal generators and optical multimeters.
In an embodiment of the invention, a tunable laser with a base, a gain medium, a tunable feedback device and a first compensating element is disclosed. The gain medium and the tunable feedback device are coupled to the base. The tunable feedback device provides feedback of a selected wavelength to the gain medium. The first compensating element provides coupling to the base for at lease one of the gain medium and the tunable feedback device such that thermal expansion of the compensating element maintains a substantially constant integer number of half-wavelengths within a resonant cavity defined by the gain medium and the tunable feedback device during temperature variations in the tunable laser.
In another embodiment of the invention, a tunable laser with a base, a gain medium, a first and second feedback device, a pivot arm and a first compensating element is disclosed. The gain medium, and first and second feedback devices are coupled to the base. The first feedback device provides feedback of a selected wavelength to the gain medium. The pivot arm has a proximal and distal end. At the proximal end the pivot arm pivotally attaches to the base at a first pivot axis. The second feedback device couples to the distal end of said pivot arm to provide feedback of the selected wavelength to said first feedback device. The second feedback device together with the first feedback device and the gain medium define a resonant cavity. The second feedback device responds to the arcuate displacement of the pivot arm to vary the selected wavelength. The first compensating element couples the second feedback device to the distal end of said pivot arm such that thermal expansion of the first compensating element maintains a substantially constant integer number of half-wavelengths within the resonant cavity.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.